Examining the MJO–QBO Relationship in a GCM with a Nudged Stratosphere

Recent observational results show a remarkably strong link between two hallmark modes of variability in the tropical atmosphere: the Madden-Julian oscillation (MJO) and the stratospheric quasi-biennial oscillation (QBO). The crux of the MJO-QBO relationship is a modulation of the strength of the boreal winter MJO by the QBO, with stronger MJO events when QBO winds in the lower stratosphere are easterly. Despite active research into the MJO-QBO link, the physical processes through which these two phenomena interact remain poorly understood. Hampering attempts to understand the MJO-QBO connection is the fact that many general circulation models struggle to represent an MJO-QBO relationship, making it difficult to study this question in a modeling framework and casting doubt on the robustness of the observed result.

In this work we examine whether a state-of-the-art GCM with reasonable representations of both the MJO and QBO can capture the MJO-QBO link. We demonstrate that, in free-running mode, the modeled MJO-QBO connection is weaker than the observed and not statistically robust. However, the signals associated with the model’s QBO are weaker than observed around the tropical tropopause. This bias could explain the lack of simulated QBO influence on the MJO. To resolve this and examine more directly whether the stratosphere modulates the MJO, we conduct a series of experiments in which the model’s stratospheric winds are nudged to ensure a better representation of the QBO in the tropopause region. We then examine whether these nudged simulations capture the MJO-QBO link. We also present plans to expand this preliminary work to include several other models, including forecast models and another free-running general circulation model, in a more coordinated intercomparison which examines whether models with nudged stratospheric states capture any effect of the QBO on the MJO.